I seem to have started an argument on my motorcycle forums. Here’s the scenario. Imagine you have a bike, probably a sportbike, that has good enough traction at the front tire that you can brake hard enough to lift the rear tire (called an endo). I mentioned that as an assumption on my motorcycle forums, so it applies here too. Is there any way that you could have more braking power using both your front and rear brakes than just the front? It seems to me that you’d want to use the front brakes to the point that the rear is basically weightless, but not in the air, and using the rear brakes would be pointless since there’s no weight on that tire. They insist that you wouldn’t necessarily get the most effective braking by using so much front brake that 100% of the weight of the bike is on the front. I’m looking for a scentific answer from someone who knows what they’re talking about.
By the way, you can assume anything reasonable about the bike, such as which position the rider is in, how good of traction the rear tire gets (I said on the other forum it could be a drag slick for all I care), as long as the front tire still has enough traction to lift the rear tire instead of locking up.
As long as the tire compounds and their coefficients of friction are the same, front and rear, it won’t matter how the weight is distributed as long as the tires are braked just short of the slipping point. Braking force depends on wieght (normal force) and coefficient of friction.
There is no slipping point for the front tire. It just brakes harder and harder until the bike starts to flip forward. And don’t assume that the compounds are the same. The only assumption that can be made is that the front tire has good enough traction not to slip before flipping the bike.
Someone on the other forums said something I can’t argue with. They said appying the rear brakes would put a torque on the swingarm, loading the suspension, which would lower the center of gravity and allow more front brake to be used without endoing. It may not happen under all circumstances, but I can’t find a reason why it wouldn’t under the right circumstances. I hate being wrong… Can anyone refute this, or am I the fool this time?
I went with what was said in your OP as to what I could assume. My point was that maximum braking force is fairly constant regardless of the weight distribution, provided that sufficient braking effort is being applied to each of the wqheels to put them each in a near-skid.
But one of my stated assumptions was that the front tire won’t skid. A sportbike is like a bicycle. If you grab too much front brake on dry asphalt, the bike will most likely flip forward. The front tire won’t be at near-skid until the bike is sliding on the windscreen. At that point, maximum braking power is the least of your worries.
Not if you keep your weight back, it won’t. I’ve seen my brother sit back, grab a ton of brake, and roll happily down the street with his front wheel locked up.
You could, in theory, do this on any bike, though most cruisers and such I imagine it would be hard to keep on the front tire.
The front brake supplies around 70-80% of the braking power, while the rear supplies the rest. If you were to over do the front you can still use the rear. I would think that unless you are trying to do an endo on most bikes you wouldn’t. Usually you can tell if you’ve gotten to much front brake because it jerks a bit, unless of course you’re already high siding. I know that even if I use too much front my rear is still on the ground and I can use both brakes.
I can’t imagine having 100% of the weight on the front tire, that would mean either an endo, which I would hope that’s what the rider wanted, or you’re in the middle of a high side. I’m pretty sure, though I don’t go around doing endos, that you have to actually try and do one, I’m guessing that it’s easier to do on a sport bike.
I think I’m a bit confused though about what you want. Unless you have been trying to get the rear tire up I don’t think most are going to. I’ve never really riden a sport bike, but I can’t imagine a bike where the rear would lift unless you wanted it to. If that’s what you’re trying to do then yes, there wouldn’t be any point in using the rear brake at all, but you will not be able to stop as fast as someone using both brakes if everything else is equal.
This is a good question and is deeper than first appears. This is frequently discussed in various motorcycle safety courses. The classic answer is using both front and rear brakes yields the shortest stopping distance. They usually draw it as horizontal bar graphs, with front+rear braking producing the shortest graph.
However I’m not sure of the scientific accuracy of that or where the data came from. Almost certainly it’s a generalization that doesn’t apply in all cases to all bikes.
If you watch motorcycle road races, you’ll often see professional racers braking so hard into a corner that the rear tire is slightly off the ground. If using front and rear brakes and avoiding the rear lift was better, they’d probably do that – but they don’t.
The “front+rear is superior” idea probably comes from older data on older bikes and tires. In those cases the front tire couldn’t generate enough friction to lift the rear, when combined with wheelbase and other issues. Today’s sport bikes have vastly superior rubber and shorter wheelbases, making doing a “stoppie” (raising the rear) quite easy.
That said, generalization is impossible. Here’s why: bikes differ widely in wheelbase, rubber compound and braking systems. A long wheelbase bike with harder-compound touring tires will probably stop shorter using front+rear breaking. A sport bike with high-perf street tires (e.g, Michelin Pilot Power 2CT) will probably stop shorter using just front braking. You could use the rear, but the best deceleration is probably when the rear is almost totally unloaded so rear braking is almost immaterial no matter whether you apply it or not.
Another complication: many bikes nowadays used linked brakes so applying front automatically applies a certain % of rear brake. Increasingly those bikes also have ABS brakes on front and rear.
You could do tests in a parking lot. Try braking from a safe speed on a sport bike using just the front, then front+rear. If front+rear is better, then the same rider on the same bike should be able to consistently stop shorter using that method. For a modern sport bike with high perf rubber, I think you’ll lose more by reducing front braking to keep the rear firmly planted than the rear braking will contribute.
This illustrates SnailBoy’s question. Why would the best rider in the world do this many times each race if keeping the back wheel on the ground produced better braking? It’s likely not because he’s not paying attention but because the stoppie technique (or approximating that by mostly unloading the rear tire) produces optimal deceleration.
If that’s wrong, maybe one of Rossi’s competitors will read this and realize how easy it is to beat him by just using their back brake. I can see the headlines now: “Rossi unseated by competitors using novel braking strategy obtained from Straightdope.com.”
I think that you are correct - that applying the front brake just to the point of a “stoppie” (if that is possible with a given bike, which you said it was for the purposes of this question) will result in the shortest possible braking distance. Think of it as a moment balance. The deceleration reaction from the front contact patch times the height of the c.g. will equal the weight of the bike times the horizontal distance between the c.g. and the front contact patch. This also means that you want to keep the bike nearly horizontal - no sense letting the back tire come up a bunch, because it hurts the c.g. location w.r.t. applying maximum deceleration forces.
Since sliding friction is lower than static friction for tires (neglecting for the moment that some relative motion exists near the break-away point), you don’t want to just grab a whole handful (and footful) of brakes, because locking up the wheels will result in longer stopping distances. Best stopping will be to use the front brake until either a stoppie would result (e.g. sportbike), or it would be just short of locking up (e.g. cruiser). Then, if there is still weight on the rear wheel, to use the rear brake until it was just short of locking.
For practical reasons, a stoppie (as outlined in the OP) is probably not the shortest way to stop in real life. You’d probably like to keep the rear tire at least lightly loaded - then by applying the rear brake, you can tell that you are near endo-ing when you feel the back tire start to skid. At that point, let up on the front brake a fraction, and keep adjusting until you’ve decelerated enough (to stop, turn, or whatever). Hope that’s clear.
On preview - the best stopping performance comes from being the best rider in the world.
And by that I mean, rather than feel for rear tire lock-up, to sense (and react to) the rotation of the bike as it gets near endo-ing. For that, the reaction times are much shorter (because the endo rate is non-linear) and the required brake modulation is much more sensitive. A skill for experts (or immortals) only, as mentioned.
A lot of good information, but no absolute answer. I thought for sure using just the front brakes to the point that the rear tire is floating would be the best way, until someone mentioned the rear brakes torquing the swingarm and lowering the bike. What do you guys think of that? With a lower center of gravity, you could use more front braking, or maybe the same front braking with the additive effect from the rear braking. Personally, I don’t think it will lower the center of gravity enough on a sportbike to make it worth getting off the front brakes, but I can’t see a definite reason why it wouldn’t help. The empirical evidence (that picture of Rossi) suggests that on sportbikes (or MotoGP bikes at least), you just need the front brake, so I guess I’ll go with it unless someone can add something.
What about the opposite situation? In a drag race, it is fortuitous to have all the weight on the rear tire for the first 100 feet or so. Maximum traction and such.
I own a sandrail (vw powered, stripped down dune buggy) and it has no front brakes at all. Nothing. Why not? The front end is so light, brakes would have no effect. I do however have independent control of the rear wheels, and can spin the thing around like a tank. The front wheels just keep the front end from dragging on the ground. (and throw dirt on the occupants, “Here are some goggles, and keep your mouth closed.”)
I can’t see how it would matter much which tire is doing the braking, provided the rider still has control of the bike, and said wheel has sufficient weight on it.
I thought an absolute answer was provided: “applying the front brake just to the point of a “stoppie” (if that is possible with a given bike, which you said it was for the purposes of this question) will result in the shortest possible braking distance.”
If there is any noticeable effect in slowing the rear tire and giving a back-reaction to the swingarm and thus lowering the rear ride height, then it should be immaterial how much weight is actually carried by the back end. Do your stoppie like a GP rider, and if you want to ride the rear brake, then go ahead - but you also don’t want a speed mismatch when you come back down or it could unsettle the back end. And I can’t see this being much of an effect, if at all - these bikes should be fairly stiffly suspended, and the back wheel is not all that heavy compared to the vehicle+rider weight. Hope that helps.
That would only be on bikes where a stoppie is easy to do. Most bikes will not do them, or you’re really going to have to try.
I had another post last night then we lost power, I hope I can remember it.
While the picture that’s above does show at least one person doing it, do race bikes still have rear brakes? If so why? If all you need is the front, then why not take away the extra weight and possible drag of the rear brake? I don’t watch motorcycle racing, since we don’t get it where I live so I know very little about it. But wouldn’t that make it harder to make the turn with the rear wheel up? I mean the rear wheel is what makes the bike turn so you have to wait that split second for the rear to drop. I’m guessing the riders that do it are willing to sacrafice the split second.
I still think that on the majority of bikes in most situations it’s best to use both brakes, otherwise I think you’d hear about it a lot more.
Grabbing a hadful of front brake and trying a stoppie is likely to end up unfortunately.
The problem comes when the forks hit the lower limit of travel and the descending weight of the machine causes a jolt, which will either break the front wheel grip, or the sudden application of this force may cause loss of control.
Stoppies should be done by smoothly braking ever harder which allows the forks to compress without that shock on the limits.
Seems to me that the pros might lift the rear wheel frequently during braking since avoiding this would require leaning back such that when they got back on the throttle they’d have to worry about flipping backward.
The problem with your question is that it relies on not skidding. That’s just not practical. Technically, traction should not depend on the size of the contact patch contacting the ground: a small contact patch has the same traction as a large contact patch since the additional pressure cancels out the smaller area, i.e, a contact patch half the size as another has twice the psi pressing on the ground as the larger one . This is physics 101…technically. This means dragsters don’t need “slicks” to get the most traction. Yet, this plainly isn’t the case. Why? Because a narrower tire’s tiny contact patch wouldn’t be able to handle the shear forces without shredding and high temperatures without burning. So, wider tires are used to spread out the stress. Ultimately, this is a “strength of materials” issue.
However, you may be correct on dirt. Forcing all of the weight on the front might increase braking performance because the knobby tires dig in more, a phenomenon not found on pavement.
For race bikes, not all braking is done in a straight line - you definitely want that rear brake. Even if it were, there have been enough other benefits alluded to above to recommend a rear brake.
Keep in mind that any rearward weight shift is subtle, and they don’t go from full-stop to full-go - they are going to crank the bike over in most cases and feed in the throttle gradually. For a bicycle, where the rider is the vast majority of the mass, in a panic stop you will decelerate much quicker by shifting the weight back as much as practicable.
Incorrect. Actually, the physics of tire-to-pavement contact is not Coulomb friction (which is what you’ve described), where the maximum load is proportional to normal force, but is dependent on deformation of the tire into pavement irregularities and localized shear forces. This is why high-grip tires have larger cross-sections and/or contact patches, not (primarily) to preclude large-scale shear-induced failure of the rubber, but to allow for more rubber-to-road contact area. Note though, as you’ve alluded, making a tire’s contact area larger allows a softer compound for the same wear characteristics, and this softer compound will conform to the surface better giving higher grip. This is actually the “phenomenon found on pavement”. Hope this helps.
Here is a cite, if you want much more, I’ll have to dig harder:
wiki (stub): “Conventional pneumatic tires do not behave as classical friction theory would suggest”
I’ve seen my brother sit back, grab a ton of brake, and roll happily down the street with his front wheel locked up.